1. Field of the Invention
The present invention relates to a magneto-optical recording medium capable of recording, reading and erasing of information, particularly to such medium compatible to visible short-wavelength laser beams, particularly 690 nm or less.
2. Description of the Related Art
Amorphous thin films of rare earth-transition metal alloys which have conventionally been used as the magnetic materials for magneto-optical discs involve a problem of low readout signal levels because of their small Kerr rotation angles. For the purpose of improving the readout signal level, a dielectric layer such as of SiO or SiO.sub.2 and a metallic reflective layer are provided on the behind of a recording medium, as disclosed in Japanese Unexamined Patent Publication No. 12428/1982. The cross section of such recording medium is shown in FIG. 1. The structure of this recording medium will hereinafter be referred to as reflective layer structure, in which 91 is a substrate such as of glass, 92 a magnetic layer, 93 a SiO.sub.2 thin layer, and 94 a metallic layer. In this medium, the Kerr rotation angle changes by varying the film thickness of the SiO.sub.2 thin layer 93, as shown in FIG. 2, Accordingly, the Kerr rotation angle can be increased by suitably selecting the film thickness of the SiO.sub.2 thin layer.
Meanwhile, as shown in FIG. 3, an improved reflective layer structure is discussed (Journal of Appl. Mag. Soc. Jpn. Vol. 11, No. 2, 1987). This improved structure additionally has a dielectric layer 112 interposed between the substrate 91 and the magnetic layer 92 in the reflective layer structure of FIG. 1. In the improved structure, since the magneto-optical effects of the light reflected on the surface of the magnetic layer and the light transmitted through the magnetic layer and reflected on the reflective layer are utilized so as to increase the Kerr rotation angle, the film thickness of the magnetic layer is as thin as 20 nm to 30 nm. In this structure, a second dielectric layer having a film thickness of 20 nm, which is thinner than the first dielectric layer (80 nm), is provided. It is reported that the Kerr rotation angle at the wavelength of 780 nm in this case is 1.5.degree. and that the reflectivity is 12%, showing that the reflectivity is lowered due to the increased Kerr rotation angle. The currently available magneto-optical recording media employ such structure so as to increase the Kerr rotation angles and in turn the readout signal levels.
On the other hand, as an improvement of the recording layer itself, an exchange-coupled double-layer film consisting of a layer (reading layer) having a high Curie temperature and a relatively great Kerr rotation angle and a layer (writing layer) having a low Curie temperature and high recording sensitivity is proposed as the recording layer (Japanese Patent Publication No. 35371/1990). The cross-sectional structure of the exchange-coupled double-layer film shown in FIG. 4 consists of a magnetic layer 121 (reading layer) having a high Curie temperature and a small coersivity at room temperature and a magnetic layer 122 (writing layer) having not very high Curie temperature and a great coersivity at room temperature. The conventional record mark formation process in the exchange-coupled double-layer film will be described referring to FIG. 5. As shown in FIG. 5(a), upon irradiation of a laser beam 131 onto the recording layer, the temperature of the irradiated portion is elevated. Since the reading layer 121 has a high Curie temperature, magnetization-of reading layer cannot be disappeared, but magnetization of the writing layer 122 having not very high Curie temperature can be disappeared. If an external magnetic field 132 is applied to he magnetic layer as such, the magnetization of the reading layer 121 is reversed, as shown in FIG. 5(b). As the temperature drops, the magnetization in the reading layer 121 is transcribed to the writing layer 122 by the exchange-coupling, as shown in FIG. 5(c). In such recording process, it is difficult to form net record marks in the reading layer 121 having a high Curie temperature and a small coersivity, leading readily to increased record noise levels.
In such exchange-coupled double-layer film, incorporation of the reflective layer structure, in which the film thickness of the magnetic layer is reduced, is discussed (Japanese Unexamined Patent Publication No. 230535/1990), and the incorporation of such structure is considered to have effects of increasing the Kerr rotation angle and of moderating the thermal interference. The medium consists of a PC (polycarbonate) substrate, with a 80 nm-thick AlSiN dielectric layer, a 15 nm-thick first magnetic layer (Gd.sub.0.23 Fe.sub.0.73 CO.sub.0.04), a 15 nm-thick second magnetic layer (Tb.sub.0.22 Fe.sub.0.72 CO.sub.0.08), and 25 nm-thick AlSiN dielectric layer and a 40 nm-thick Al reflective layer being laminated successively in this order. In this constitution, the total film thickness of the recording layer mounts to 30 nm, and the optical enhancement condition thereof is absolutely the same as in the said improved reflective layer structure. Accordingly, although the Kerr rotation angle can be increased due to the optical enhancement, the problem that the reflectivity is lowered has not yet been cleared.
Further, the problem that the record noise level tends to be increased in the double-layer film has not yet been cleared either.
As described above, in the conventional media having the reflective layer structure, the thickness of the recording layer is reduced so as to obtain a high C/N ratio, and the light reflected on the recording layer and the light transmitted through the recording layer and reflected on the metallic layer are utilized to increase the Kerr rotation angle and magnify the readout signals. The C/N ratio of the magneto-optical disc is characterized by the figure of merit (.theta..sub.k R.sup.1/2), and it has been considered that the C/N ratio is predominantly dependent on the Kerr rotation angle and that the reflectivity may sufficiently be ten-plus %.
However, as red laser beams having wavelengths of 670 nm to 690 nm have recently been put into practical uses and short-wavelength light sources are actually introduced, a problem manifested that the noise in the circuit system is extremely increased compared with other noises due to the reduction in the radiation sensitivity of the photo-detector. Accordingly, a medium structure which can provide a high reflectivity of 15% or more and a great Kerr rotation angle, in the short wavelength region of visible light and of shorter wavelength is desired.
Although a magneto-optical medium of the reflective layer structure, in which the film thickness of the exchange-coupled double-layer magnetic film is reduced, has been discussed, the medium having such structure involves a problem that if a thin magnetic layer is employed so as to enhance the Kerr rotation, the reflectivity is lowered. Further, since the readout signals depend on the record marks in the reading layer, it has been difficult to form marks which is free from distortion as much as possible in the reading layer having a high Curie temperature or to control the marks so as to minimize the noise. Besides, since the film thickness of the layer having a great Kerr rotation angle in the exchange-coupled film is as thin as 15 nm, the Kerr rotation angle cannot fully be increased, disadvantageously.